Combine flour, spices and baking soda in small bowl. Beat butter and sugars in large bowl until creamy. Beat in pumpkin, egg and vanilla until blended; gradually beat in flour mixture. Stir in 1 1/2 C. of the white chips and the nuts. Spread into a greased 15 1/2x10 1/2 jelly roll pan. Bake in a preheated 350 degree oven for 18-20 minutes until toothpick comes out clean. Cool in pan on wire rack.
Place remaining white chips in heavy duty plastic bag. Microwave on high for 45 seconds; knead. Microwave at 10 second intervals, kneading until smooth. Cut tiny corner from bag; squeeze to drizzle over the bars.